This invention relates to a speed varying sprocket assembly, and particularly to a sprocket tooth construction of a sprocket freewheel assembly for a vehicle such as a bicycle.
Speed variable bicycles are now popular since they are adaptable for being operated at different geographical conditions. A typcial speed variable bicycle includes a sprocket freewheel assembly having a plurality of sprockets with different numbers of teeth mounted coaxially on a rear wheel assembly thereof, and a derailleur which is connected to an operating handle by means of a cord. By operating the handle, a transmission chain can be shifted from one of the sprockets to another, that is to say, the chain can be raised from a sprocket of smaller number of teeth to another sprocket of greater number of teeth or it can be lowered to a sprocket of more teeth to another sprocket of fewer teeth. As shown in FIG. 1, the teeth 5 of a conventional sprocket 4 usually has a tooth tip 6 with a rectangular cross-section; in other words, with a cross-section having a uniform circumferential length and a uniform axial width. In shifting a transmission chain from one of the wheels to a next wheel, such a tooth tip construction does not permit a rapid release of the chain from the teeth of the former wheel or a rapid engagement of the chain with the teeth of the next wheel, thereby requiring a long path of rotation to complete the chain shifting operation. In some cases, the chain can not be shifted to the next wheel smoothly, thus causing undesirable friction to the teeth. Moreover, due to the above mentioned tooth tip construction, it is somewhat difficult to manipulate the operating handle and the derailleur so as to set them in a desired position.